The solar cells of photovoltaic modules are typically fabricated as separate physical entities with light gathering surface areas on the order of 4-6 cm2 or larger. For this reason, it is standard practice for power generating applications to mount photovoltaic modules containing one or more solar cells in a flat array on a supporting substrate or panel so that their light gathering surfaces provide an approximation of a single large light gathering surface. Also, since each solar cell itself generates only a small amount of power, the required voltage and/or current is realized by interconnecting the solar cells of the module in a series and/or parallel matrix.
A conventional prior art photovoltaic module 10 is shown in FIG. 1. A photovoltaic module 10 can typically have one or more photovoltaic cells (solar cells) 12a-b disposed within it. Because of the large range in the thickness of the different layers in a solar cell 12, they are depicted schematically. Moreover, FIG. 1 is highly schematic so that it represents the features of both “thick-film” solar cells 12 and “thin-film” solar cells 12. In general, solar cells 12 that use an indirect band gap material to absorb light are typically configured as “thick-film” solar cells 12 because a thick film of the absorber layer is required to absorb a sufficient amount of light. Solar cells 12 that use a direct band gap material to absorb light are typically configured as “thin-film” solar cells 12 because only a thin layer of the direct band-gap material is needed to absorb a sufficient amount of light.
The arrows at the top of FIG. 1 show the source of direct solar illumination on the photovoltaic module 10. Layer 102 of a solar cell 12 is the substrate. Glass or metal is a common substrate. In some instances, there is an encapsulation layer (not shown) coating the substrate 102. In some embodiments, each solar cell 12 in the photovoltaic module 10 has its own discrete substrate 102 as illustrated in FIG. 1. In other embodiments, there is a substrate 102 that is common to all or many of the solar cells 12 of the photovoltaic module 10.
Layer 104 is the back electrical contact for a solar cell 12 in photovoltaic module 10. Layer 106 is the semiconductor absorber layer of a solar cell 12 in photovoltaic module 10. In a given solar cell 12, back electrical contact 104 makes ohmic contact with the absorber layer 106. In many but not all cases, absorber layer 106 is a p-type semiconductor. The absorber layer 106 is thick enough to absorb light. Layer 108 is the semiconductor junction partner that, together with semiconductor absorber layer 106, completes the formation of a p-n junction of a solar cell 12. A p-n junction is a common type of junction found in solar cells 12. In p-n junction based solar cells 12, when the semiconductor absorber layer 106 is a p-type doped material, the junction partner 108 is an n-type doped material. Conversely, when the semiconductor absorber layer 106 is an n-type doped material, the junction partner 108 is a p-type doped material. Generally, the junction partner 108 is much thinner than the absorber layer 106. The junction partner 108 is highly transparent to solar radiation. The junction partner 108 is also known as the window layer, since it lets the light pass down to the absorber layer 106.
In a typical thick-film solar cell, absorber layer 106 and window layer 108 can be made from the same semiconductor material but have different carrier types (dopants) and/or carrier concentrations in order to give the two layers their distinct p-type and n-type properties. In thin-film solar cells in which copper-indium-gallium-diselenide (CIGS) is the absorber layer 106, the use of CdS to form junction partner 108 has resulted in high efficiency cells. Other materials that can be used for junction partner 108 include, but are not limited to, In2Se3, In2S3, ZnS, ZnSe, CdlnS, CdZnS, ZnIn2Se4, Zn1-xMgxO, CdS, SnO2, ZnO, ZrO2 and doped ZnO.
In a typical thick-film solar cells 12, the absorber layer 106 and the window layer 108 can be made from the same semiconductor material but have different carrier types (dopants) and/or carrier concentrations in order to give the two layers their distinct p-type and n-type properties. In thin-film solar cells 12 in which copper-indium-gallium-diselenide (CIGS) is the absorber layer 106, the use of CdS to form the junction partner 108 has resulted in high efficiency photovoltaic devices. The layer 110 is the counter electrode, which completes the functioning solar cell 12. The counter electrode 110 is used to draw current away from the junction since the junction partner 108 is generally too resistive to serve this function. As such, the counter electrode 110 should be highly conductive and transparent to light. The counter electrode 110 can in fact be a comb-like structure of metal printed onto the layer 108 rather than forming a discrete layer. The counter electrode 110 is typically a transparent conductive oxide (TCO) such as doped zinc oxide. However, even when a TCO layer is present, a bus bar network 114 is typically needed in conventional photovoltaic modules 10 to draw off current since the TCO has too much resistance to efficiently perform this function in larger photovoltaic modules. The network 114 shortens the distance charge carriers must move in the TCO layer in order to reach the metal contact, thereby reducing resistive losses. The metal bus bars, also termed grid lines, can be made of any reasonably conductive metal such as, for example, silver, steel or aluminum. The metal bars are preferably configured in a comb-like arrangement to permit light rays through the TCO layer 110. The bus bar network layer 114 and the TCO layer 110, combined, act as a single metallurgical unit, functionally interfacing with a first ohmic contact to form a current collection circuit.
Optional antireflective coating 112 allows a significant amount of extra light into the solar cell 12. Depending on the intended use of the photovoltaic module 10, it might be deposited directly on the top conductor as illustrated in FIG. 1. Alternatively or additionally, the antireflective coating 112 may be deposited on a separate cover glass that overlays the top electrode 110. Ideally, the antireflective coating 112 reduces the reflection of the solar cell 12 to very near zero over the spectral region in which photoelectric absorption occurs, and at the same time increases the reflection in the other spectral regions to reduce heating. U.S. Pat. No. 6,107,564 to Aguilera et al., hereby incorporated by reference herein in its entirety, describes representative antireflective coatings that are known in the art.
Solar cells 12 typically produce only a small voltage. For example, silicon based solar cells produce a voltage of about 0.6 volts (V). Thus, solar cells 12 are interconnected in series or parallel in order to achieve greater voltages. When connected in series, voltages of individual solar cells add together while current remains the same. Thus, solar cells arranged in series reduce the amount of current flow through such cells, compared to analogous solar cells arranged in parallel, thereby improving efficiency. As illustrated in FIG. 1, the arrangement of solar cells 12 in series is accomplished using interconnects 116. In general, an interconnect 116 places the first electrode of one solar cell 12 in electrical communication with the counter-electrode of an adjoining solar cell 12 of a photovoltaic module 10.
Various fabrication techniques (e.g., mechanical and laser scribing) are used to segment a photovoltaic module 10 into individual solar cells 12 to generate high output voltage through integration of such segmented solar cells. Grooves that separate individual solar cells typically have low series resistance and high shunt resistance to facilitate integration. Such grooves are made as small as possible in order to minimize dead area and optimize material usage. Relative to mechanical scribing, laser scribing is more precise and suitable for more types of material. This is because hard or brittle materials often break or shatter during mechanical scribing, making it difficult to create narrow grooves between solar cells.
Despite the advantages of laser scribing, problems are known to occur when scribing photovoltaic modules. For example, one method of scribing a long cylindrical photovoltaic module is to place the module horizontally and rotate it while having a stationary scriber make the cuts. However, in this arrangement the photovoltaic module is only supported at the ends and not in the middle. Gravitational effects create a “bow” effect where the middle portion of the photovoltaic module is slightly bent, creating a shape like a curved rod. This bow may not be significant, but it is enhanced when the photovoltaic module is rotated during scribing. While the photovoltaic module rotates, the bow effect creates a difference in distance between the circumference of the photovoltaic module and the stationary scriber varies as the photovoltaic module is rotating. This results in an uneven cut in the photovoltaic module since the scriber is very sensitive to changes in distance. Scribing some layers of the photovoltaic module requires precision control of the cuts. Uneven cuts could destroy the functionality of the solar cells produced by such scribing. For example, it may be intended to scribe a groove through the entirety of a layer on the photovoltaic module. If the distance between the scribe and the photovoltaic module changes during scribing, portions of the groove may not be deep enough to cut completely through the layer.
Also, the photovoltaic module is normally spun at a high rotational speed for portions of the scribing process. Imperfections in the shape of the photovoltaic module, including the bow effect, create a non-symmetrical moment of inertia as the photovoltaic module rotates. Thus, the photovoltaic module experiences an uneven outward pull due to the centrifugal force. This enhances the undesired shape of the bow, resulting in even larger variances in distance between the cell and the scriber during rotation. For example, a distance change of three millimeters (mm) between the surface of the photovoltaic module and the scriber during rotation could result in fatal defects in the design of the solar cells of the photovoltaic module. Conventional mechanical and laser scribers cannot adjust well to the changes in distance between the scribe and the photovoltaic module. A change in distance results in an uneven force being applied as the photovoltaic module is rotationally scribed, resulting in differences in width and depth of the grooves cut by the scribe.
A mechanical scriber for scribing solar cells is described in U.S. Pat. No. 4,502,255 (hereinafter “Lin”). The downward force of the Lin scriber can be controlled to a precise amount. However, the Lin scriber is designed only to work for planar photovoltaic modules. The Lin scriber cannot readily be used to scribe non-planar photovoltaic modules.
Given the above background, what is needed in the art are systems and methods for scribing any elongated objects, such as non-planar (e.g., cylindrical) photovoltaic modules, that are subject to the bow effect. Such systems and methods can be, for example, used to form solar cells in an elongated photovoltaic module such that a constant force cut is provided regardless of the position of the scriber along a long dimension of the photovoltaic module.
Discussion or citation of a reference herein will not be construed as an admission that such reference is prior art to the present application.